1. Field of the Invention
The present invention relates to data storage apparatus for magnetically read and writing information on data storage media. More particularly, the invention concerns the fabrication of suspension assemblies designed to carry read/write heads in magnetic disk drive storage devices.
2. Description of the Prior Art
By way of background, a read/write head of a magnetic disk drive storage device (“disk drive”) is typically incorporated on an air bearing slider that is designed to fly closely above the surface of a spinning magnetic disk medium during drive operation. The slider is mounted at the end of a suspension assembly that in turn is cantilevered from the arm of a pivotable actuator. When energized, the actuator sweeps the suspension across the disk surface, allowing the read/write head to read and write data in a series of concentric tracks.
The suspension of a conventional disk drive typically includes a relatively stiff load beam whose base end (known as the “mount plate”) is attached to the actuator arm and whose free end (known as the “functional end”) mounts a flexure that carries an associated slider and its integrated read/write head in a gimbaled configuration. Disposed between the mount plate and the functional end of the load beam is a “hinge” that is compliant in the vertical bending direction (normal to the disk surface). The hinge enables the load beam to suspend and load the slider and the read/write head toward the spinning disk surface. It is then the job of the flexure to allow the read/write head to pitch and roll in order to adjust its orientation for unavoidable disk surface run out or flatness variations.
The foregoing suspension components are quite small. A typical suspension is about 18 mm in length. The load beam typically has a thickness of between about 0.03-0.1 mm and the flexure typically has a thickness of between about 0.02-0.03 mm. The slider is typically about 1.25 mm long×1.00 mm wide×0.30 mm thick, and the read/write head carried thereon is a fraction of that size.
A design requirement of a disk drive suspension load beam is that it be sufficiently compliant in the vertical bending direction to facilitate proper gram loading of the slider and read/write head relative to the supportive air bearing force. At the same time, the load beam must be relatively stiff in the horizontal direction (parallel to the disk surface) to prevent off-track sway misalignment. It must also be torsionally stiff to prevent off-track rotational misalignment. In addition to these static structural requirements, the load beam must have good dynamic characteristics to prevent unwanted vibration and flutter. Excessive gain caused by resonance at critical dynamic frequencies can induce unwanted torsion, sway and bending, all of which can contribute to track misregistration problems, excessive noise, and undue wear. Dynamic design considerations have become particularly acute as recording density and TPI (Tracks Per Inch) requirements continue to increase. This has necessitated higher track servoing bandwidths, which in turn has established a need for higher dynamic performance suspensions.
Historically, suspension load beams have been fabricated by combining several stainless steel sheet stock elements to form the mount plate, the hinge and the functional end. In some designs, a single sheet of stainless steel has stainless steel pieces welded to it to develop the required thicknesses for the mount plate and the functional end. The hinge is then defined by the initial sheet material that lies between the welded pieces. In other designs, the mount plate, the hinge and the functional end are assembled from three different pieces of stainless steel sheet stock that are welded together.
A disadvantage of welded load beam designs is that welding adds more processing steps and can introduce thermal distortions. Individual thin material hardness is also difficult to control. These conditions lead to flatness variations relative to the principal plane of the load beam. Flatness is an important parameter to control because a non-flat load beam profile can cause suspension flutter due to air flow at operational disk rotation speeds. Welding also tends to reduce the real estate available for components such as piezoelectric milliactuators or the like. There are also free vibrating lengths of material between the weld points that contribute to dynamic flutter and mode gains at critical frequencies, thereby adversely affecting performance.
In recent years, manufacturers have begun using partial etch processes to produce disk drive suspensions as an alternative to welded constructions. According to this approach, fabrication begins with a sheet of stainless steel sheet stock that is rolled to a desired thickness using a rolling reduction technique. Photo-chemical partial etching is then employed to form areas of reduced thickness in the rolled material, such as the hinge section. In addition, partial etched pockets can be formed to reduce load beam mass and inertia without sacrificing the required static and dynamic stiffness characteristics.
In general, the use of photochemical etch processing allows load beams to perform much better than conventionally formed load beams that have not been etched. This approach has also been found to offer a great deal of design freedom because many elaborate pocket geometries can be formed, thereby allowing dynamic characteristics to be fine-tuned by distributing load beam mass and stiffness in strategic fashion.
Notwithstanding its advantages, photochemical etching generates excessive tolerances in the vertical direction normal to the principal plane of the load beam. Such tolerances can be 2-4 times that of the rolled starting material. This can produce unacceptable variations in gram loading and torsional dynamic characteristics. The problem is that the tolerances required to produce satisfactory pocket depth uniformity are at the process limits of photo-chemical etching. Although the depth of material removed is substantially a linear function of the length of time the metal is exposed to the chemical etching solvent, there are a number of variables that affect the ability to precisely control the amount of metal removed. Such variables include temperature, chemical contamination, chemical solvent concentration, impurities in both solvent and metal, and initial metal thickness.
A proposed solution to this problem is discussed in commonly-assigned U.S. Pat. No. 6,215,622 (the “'622 patent”). According to the '622 patent, a suspension member is formed as a laminate that includes at least two metallic material sheets with an etch retardant layer between the metallic sheets. The etch retardant layer acts as an etch stop when etching one or both of the metallic sheets, thereby controlling the amount of material removed and preserving thickness tolerances within acceptable levels. Although the technique disclosed in the '622 patent represents an important step forward in the design of etched suspension structures, it is believed that further improvements can be realized in the fabrication of such components. In particular, it should be noted that the '622 patent proposes a construction wherein either a deposition process (for a metallic etch stop layer) or an adhesive bonding process (for a thermoplastic etch stop layer) is used to apply the etch stop layer to one of the metallic sheets. Thereafter, the etch stop layer-coated metallic sheet is rolled against another metallic sheet in a cladding operation, thereby bonding the sheets together. Fabrication of the suspension components of the '622 patent thus involves at least two separate fabrication operations; namely, a deposition or bonding operation followed by a cladding operation. It should also be noted that the etch stop layer is very thin. For example, if the etch stop layer comprises a metal, it is ideally only a few atoms thick. If the etch stop layer is a thermoplastic material, it will be about 1-7 microns in thickness. In either case, the etch stop layer is not intended to perform a load bearing function in the suspension member.
It is submitted that a new manufacturing method that improves upon the above-described suspension construction techniques is warranted. What would be particularly desirable is a manufacturing method that allows partially etched suspension components to be formed without the attendant disadvantages of the above-described prior art technique.